Complications from arterial diseases, such as myocardial infarctions (MI) and cerebral vascular accidents (CVA), cause a majority of the deaths in the western world. The survival of a patient is highly dependent upon the early diagnosis and treatment of stenotic lesions in blood vessels, which are characteristic of these diseases. Dye contrast x-ray angiography has long provided an invasive means of imaging the blood vessels. However, this procedure is disadvantageous due to the invasiveness of the procedure and the exposure of the patient to potentially harmful ionizing radiation. Magnetic resonance angiography (MRA) was initially thought to be the answer to this problem. It is a non-invasive procedure, and the evidence to date is that the high magnetic fields pose no danger to human cells. However, all MRA imaging processes in use today fail in the critical area of arterial disease detection, because massive signal loss occurs in areas of stenotic flow, thus causing an overestimation of the extent of stenoses or even causing a diagnosis of total occlusion.
Previous researchers have seen signal loss in a variety of situations having fluid flow. The observation of patients with aortic coarctations, which are constrictions in the aorta or other blood vessels, using MRA showed that high velocity flow, as seen in fluid jets through obstructive or regurgitant valves, produces a low intensity area as does turbulent flow. See Simpson et al., "Cine Magnetic Resonance Imaging for Evaluation of Anatomy and Flow Relations in Infants and Children with Coarctation of the Aorta," Circulation, v. 78 pp. 142-148 (1988). Generally, turbulent flow refers to fluid flow which is characterized by chaotic motion with various velocity, acceleration, and directional attributes. As described in the foregoing paper, the maximum jet length of fluid flow distal to a coarctation was found to correlate well with the coarctation severity measured by angiography or surgery. Other researchers warn against using this signal loss as an absolute measure of the existence of a coarctation. See Mirowitz et al., "Pseudocoarctation of the Aorta: Pitfall on Cine MR Imaging," J Comput Assist Tomogr, v. 14, pp. 753-754 ( 1990). These researchers found signal loss similar to that seen in a coarctation.
The phenomena of signal loss in magnetic resonance (MR) images is not isolated to coarctations. Researchers examining MR images of turbulent flow through straight tubes and orifice type constrictions found that transition to turbulence did not necessarily cause signal loss, as evidenced by the accurate magnitude imaging of turbulent flow through straight tubes. See Evans et al., "Effects of Turbulence on Signal Intensity in Gradient Echo Images," Invest Radiol, v. 23, pp. 512-518 (1988). However, MR images of flow through an orifice showed significant signal loss with increasing construction (i.e., high Reynolds number). The area of reduced signal intensity was shown to increase linearly with Reynolds number, while the relative signal intensity past the orifice was found to decrease linearly with Reynolds number (above the threshold value).
In Krug et al., "MR Imaging of Poststenotic Flow Phenomena: Experimental Studies," JMRI v. 1, pp. 585-591(1991), a study of MR images of flow distal to a cosine shaped model stenosis showed that the length of the region of poststenotic changes in signal amplitude and phase increased with stenosis grade at a constant flow rate and that the length of the region of poststenotic changes increased with flow rate at a constant stenosis grade. A further study of flow distal to a cosine shaped stenosis, described in Oshinski, "MRI of Stenotic Flows," PhD Thesis, Georgia Institute of Technology (1993), showed that the signal loss depended on absolute fluid velocity, rather than Reynolds number. The signal loss was allegedly attributed to acceleration gradients and turbulence. The region of signal loss was shown to decrease as the thickness of the imaged region (slice region) was increased.
Several studies have advocated the reduction of echo time as a means for reducing or eliminating signal loss when imaging fluid flow. Improvement has been shown in the imaging of tortuous cerebral vessels with the use of shorter echo times. In this regard, see Schmalbrock et al., "Volume MR Angiography: Methods to Achieve Very ShortEcho Times," Radiology, 175:861-865 (1990). One study, described in showed a marked reduction in signal loss when comparing poststenotic (orifice type stenosis) jet velocity measured with an echo time of 3.6 msec as compared to 14 msec. Kilner et al., "Valve and Great Vessel Stenosis: Assessment with MR Jet Velocity Mapping," Radiology, v. 178, pp. 229-235 (1991). Other researchers argue that the improvement in MR images of stenotic flow is caused by a decrease in magnetic gradient duration, rather than shortened echo time. For instance, see Urchuk et al., "Mechanisms of Flow-induced Signal Loss in MR Angiography," JMRI, v. 2, pp. 453-462 (1992).
In essence, the theoretical explanations for signal loss vary with the gradient sequence used. One analysis assuming only velocity encoding, described in Nalcioglu et al., "Application of MRI in Fluid Mechanics of Turbulent Flow," 7th Annual SMRM Abstracts, p. 418 (1987), and another with a partial gradient echo sequence Gatenby et al., "Measurement of Turbulent Intensity using Partial Echo Techniques," 12th Annual SMRM Conference Abstracts, p. 2914 (1992), argue that the fluctuating component of velocity in turbulent flow causes an exponential decline in signal intensity. The same phenomena is said to occur on even echoes of a multiple echo spin echo sequence. See, e.g., De Gennes, P.G. (1969) "Theory of Spin Echoes in a Turbulent Fluid," Physics Lett. 29:20-21. Others argue that signal loss using spin-echo imaging or echo-planer imaging can be caused by fluid shear stress. In regard to the former, see Kuethe et al., "Fluid Shear and Spin-Echo Images," MRM, v. 10, pp. 57-70 (1989), and in regard to the latter see Kose, "Visualization of Shear Distribution in Turbulent Fluids Using Echo-Planar Imaging," 11th Annual SMRM Conference Abstracts, p. 362 (1991).
The aforementioned studies show the universal presence of signal loss in areas of constriction, or areas in vessels characterized by both contraction and expansion at high Reynolds numbers. With increasing Reynolds number or increasing degree of constriction, the signal loss has been found to increase. This signal loss has been attributed to a host of flow parameters, including turbulence, high velocities, and high shear rate, and to a host of imaging parameters, including echo time and magnetic gradient duration. Thus, presently in the art, there is neither a consistent explanation for the reason for signal loss nor a solution for the avoidance of signal loss in MR images of turbulent fluid flow.